oxygenation
A-a = PAO₂ − PaO₂
Normal: < 10 mmHg (young); < age/4 (age-adjusted)
Normal: ~100 on room air
Normal: 80–100 mmHg
The A-a gradient measures the difference between the oxygen tension in the alveoli (PAO₂) and the oxygen in the arterial blood (PaO₂). An elevated gradient indicates a gas exchange problem: the alveolus is full of oxygen but it is not transferring efficiently into the blood. A normal gradient with hypoxemia points to hypoventilation or low inspired O₂ as the cause.
Calculate in any patient with hypoxemia to determine the mechanism. Elevated A-a = intrapulmonary problem (V/Q mismatch, shunt, diffusion impairment). Normal A-a = extrapulmonary problem (hypoventilation, low FiO₂, high altitude).
Normal young adult, room air
→ 8 mmHg
Normal A-a gradient — no gas exchange impairment
Pulmonary embolism
→ 45 mmHg
Markedly elevated — dead-space V/Q mismatch from PE
COPD patient, room air
→ 17 mmHg
Mildly elevated — some V/Q mismatch with baseline hypoventilation
Elevated A-a gradient is seen in pneumonia, PE, ARDS, pulmonary edema, atelectasis, and any condition causing V/Q mismatch or shunt. A-a gradient increases with age and with higher FiO₂. The gradient should improve with supplemental O₂ in V/Q mismatch but will NOT improve with pure shunt (blood bypasses ventilated alveoli entirely).